1. Field of Endeavor
The invention relates to the field of materials engineering. It concerns an iron-based high-temperature alloy, which contains about 20% by weight Cr and several % by weight Al, as well as small amounts of other constituents, and which has good mechanical properties and very good oxidation resistance at operating temperatures up to 1000° C.
2. Brief Description of the Related Art
For some time, iron-based ODS (oxide-dispersion-strengthened) materials, for example ferritic ODS FeCrAl alloys, have been known. On account of their outstanding mechanical properties at high temperatures, they are used with preference for components that are subjected to extreme thermal and mechanical stress, for example for gas turbine blades.
ALSTOM uses such materials for tubes to protect thermocouples, which are used, for example, in gas turbines with sequential combustion for temperature control and are exposed there to extremely high temperatures and oxidizing atmospheres.
The nominal chemical compositions are specified (in % by weight) in Table 1 for known ferritic iron-based ODS alloys:
TABLE 1Nominal composition of known ODS-FeCrAlTi alloysConstituentAddition of reactiveAlloyelements (in the formdesignationFeCrAlTiSiof an oxide dispersion)Kanthal APMRem.20.05.50.030.23ZrO2—Al2O3MA 956Rem.20.04.50.5—Y2O3—Al2O3PM 2000Rem.20.05.50.5—Y2O3—Al2O3
The operating temperatures of these metallic materials reach up to about 1350° C. They have potential properties that are more typical of ceramic materials.
The materials mentioned have very high creep rupture strengths at very high temperatures and also provide outstanding high-temperature oxidation resistance by forming a protective Al2O3 film, as well as a high resistance to sulfidizing and vapor oxidation. They have highly pronounced directional-dependent properties. For example, in tubes, the creep strength in the transverse direction is only about 50% of the creep strength in the longitudinal direction.
The production of such ODS alloys is performed by powder metallurgical processes, using mechanically alloyed powder mixtures that are compacted in the known way, for example, by extrusion or by hot isostatic pressing. The compact is subsequently highly plastically deformed, usually by hot rolling, and subjected to a recrystallization annealing treatment. This type of production, but also the material compositions described, mean, inter alia, that these alloys are very expensive.